In many applications, high output radiance and small angular divergence of the electromagnetic radiation are required for achieving key performance characteristics of the systems, such as high signal-to-noise ratios or low error signals.
Several other photonics applications, including confocal scanning microscopy, optical coherence tomography (OCT), step-and-scan lithography, materials processing, multi-photon excitation, and manipulation of micro-particles and biological cells, require the generation of electromagnetic radiation that can produce high peak irradiance distributions with small lateral dimensions at the objects located in the relative proximity. In a confocal scanning microscopy or OCT system, a narrow width laser beam is scanned in the object plane. The reconstructed details in the OCT images are based on the detection of the reflected or transmitted signals, and depend on the lateral dimensions of the scanned beam. The lateral resolution of the microscope or OCT system also depends on the width of the laser beam in the object plane. The beam is scanned across the object plane to reconstruct the spatial details of the object being tested, and therefore smaller sized beams are required to increase the resolution in scanning microscopy and OCT systems. Similarly, in optical data storage applications, the storage density depends on how tightly the information is recorded on the disk. The density of the information recorded is inversely proportional to the width of the focused laser central intensity node. In optical step-and-scan lithography applications, small-sized beams or radiation patterns are projected onto the surface of a photoresist and then lateral scans are performed across the photoresist surface. The scans are followed by subsequent lateral steps to the un-exposed areas of the photoresist across the wafer, where the photoresist exposure is repeated in the form of the lateral scans. In all of the above applications, high peak irradiance and small lateral size of the electromagnetic radiation are crucially important since they directly affect the key performance characteristics of the systems, such as the signal-to-noise ratio in reconstructed images, or the minimum feature sizes achievable during the lithographic process. It is therefore desirable to establish techniques for producing electromagnetic radiation with narrow widths and high peak irradiance distributions. In addition, the produced electromagnetic radiation often needs to be controlled in size, lateral position, divergence, direction of propagation, and shape.
A traditional technique employed in generating a high intensity small size electromagnetic field distribution is based on focusing an electromagnetic radiation with a high quality lens well corrected for aberrations. The size of the resulting pattern in the focal plane is limited primarily by diffraction. The resultant field distribution of the diffraction limited lens is called an Airy pattern, and the diameter of the central peak irradiance spot of the pattern, referred to as an Airy disk, is defined as:
                              d          Airy                =                              2.44            ⁢                                          λ                ⁢                                                                  ⁢                f                            D                                =                      2.44            ⁢            λ            ⁢                                                  ⁢            N                                              (        1        )            where λ is the wavelength of the electromagnetic radiation, D is the lens aperture diameter, and f is the focal length of the focusing lens. The ratio N=f/D is referred to in literature as the f-number of an optical system. For a given wavelength of electromagnetic radiation, a reduction in the Airy disk size produced by a diffraction limited lens can be achieved only by reducing the f-number. Unfortunately, a reduction in the f-number of a lens is associated with a progressive increase in the lens size, complexity, and cost, as well as an increase in optical aberrations due to lens imperfections. At certain f-number values of a lens, the Airy disk size reduction is no longer practical. In addition, focusing of the electromagnetic radiation provides a very limited ability to alter the shape of the electromagnetic field distribution.
Amplitude and phase masks have been employed in the past to reduce the size of an Airy disk, or a diffraction limited system response to a point source known as a point-spread function (PSF). The idea of using phase masks located at the pupil of an optical system to reduce the Airy disk width was first proposed by Toraldo di Francia in 1952. Since then, it has been demonstrated that amplitude and phase masks placed at the pupil of an optical system alter the system's PSF. Several examples of PSF distributions produced with the aid of amplitude and phase masks may be found in Y. Soskind, “Field Guide to Diffractive Optics”, SPIE Press, 2011, for example on pages 15 through 16, and 29 through 35. By an appropriate selection of the amplitude and phase mask properties, the PSF size can be reduced below the size of the Airy disk value defined by equation (1). Techniques that lead to a reduction in PSF size below that of an Airy disk value are termed as super resolution techniques.
In a similar manner, employment of a phase mask within an optical system may reduce the width of a propagating laser beam. S. Imai and S. Suzuki in U.S. Pat. No. 8,164,612 disclosed a laser scanning apparatus that employs a laser source in conjunction with a single phase mask to produce super-resolved beams with the central core width less than the width of the beam when the phase mask is not present in U.S. Pat. No. 8,164,612. The major drawback of the designs disclosed in U.S. Pat. No. 8,164,612 is that there is only a marginal reduction in the width of the produced laser beam, on the order of 10%-20%. In many applications, significantly narrower output beam widths are desirable.
The employment of phase masks to shape the PSF of an optical system or the field distribution of a laser beam may have other drawbacks. It was shown by M. Soskind et. al. that for propagating Gaussian beams, the amount of energy contained within the central core of a super resolved beam produced employing amplitude and phase masks is progressively reduced with a decrease in the beam width, while the energy contained in the diffraction lobes outside of the central core is progressively increased (see, Soskind et al. 2010).
FIG. 1 presents changes in the central core diameter, the power outside of the central core, and the power contained in the central core for an optical system with central obscurations produced by an opaque, axially-symmetric amplitude mask located at the pupil of the optical system. An increase in pupil obscuration by the amplitude mask leads to a reduction in the output central core diameter, and at the same time causes a reduction in the power contained in the central core.
Phase masks located at the pupil of the system are also used to alter the shape of the PSF. The optical path difference (OPD) introduced by phase mask structures is chosen to be equal to an odd integer j of half the wavelength λ/2 of the electromagnetic radiation:
                    OPD        =                  j          ⁢                      λ            2                                              (        2        )            
In many cases, the lowest integer value j=1 is employed, and the optical path difference introduced by the phase masks is equal to half the wavelength of the electromagnetic radiation.
FIG. 2 presents normalized PSF cross-sections for an Airy distribution, as well as for optical systems with phase masks having four different radial zone sizes. The figure shows that the shape of the central core and the diffraction rings of the resulting PSF can be altered by changing the phase mask radius.
FIG. 3 presents the relative peak values of the PSF cross-sections for an Airy distribution, as well as for optical systems with the four pupil mask radial sizes shown in FIG. 2. The peak irradiance of the Airy distribution exceeds the PSF peak values of the systems employing phase masks, as shown in FIG. 3.
The ratio of the PSF peak irradiance of an optical system, in this case employing amplitude or phase masks, to the peak irradiance of an Airy distribution is known as the Strehl ratio. FIG. 4 presents the calculated Strehl ratios for optical systems with amplitude and phase masks located at the systems' pupils as a function of the masks' radial sizes. The figure indicates that the use of amplitude or phase masks to alter the PSF shape of an optical system is associated with a reduction in the Strehl ratio. The use of amplitude or phase masks located at the pupil of an optical system to alter the PSF of the system cannot significantly enhance the peak irradiance of the resulting PSF over the peak value of the diffraction limited Airy distribution.
Optical scanning mechanisms are traditionally employed to adjust the spatial location of high peak irradiance distributions in the output plane. A combination of two independently controlled mirrors and an f-theta lens represent an example of a typical scanning mechanism. An example of a two-mirror scanning unit is disclosed, for example, in U.S. Pat. No. 6,621,628.
The addition of the scanning mechanism increases the complexity of an optical system, and may also lead to reduced output peak irradiance due to imperfections and misalignments introduced by the scanner components.
Traditional optical scanning mechanisms are based on mechanical moving components, such as the aforementioned pair of galvanometric mirrors. The response time of the mechanical scanners generally depends on the physical size of the moving parts and is limited by the inertia of the moving parts. The response time of the fastest mechanical scanners is within a range of several milliseconds, corresponding to linear scan rates of several hundreds of hertz. Acquisition time in scanning microscopy or OCT systems is proportional to the scanner response time. To reduce the acquisition time, it is therefore desirable to employ scanning techniques that do not require mechanical moving parts for their implementation. It is therefore highly desirable to establish electronically controlled scanning techniques that do not rely on mechanical moving parts for their implementation, and that are capable of producing scan rates above the rates of mechanical scanners.
In view of the foregoing, it would be desirable to provide electromagnetic radiation enhancement techniques employing amplitude and phase masks that can produce high peak irradiance distributions in the output plane and will not suffer from a reduction in the Strehl ratio of the resulting field.
It would also be desirable to provide electromagnetic radiation enhancement techniques employing amplitude and phase masks that can produce high peak irradiance output distributions with small lateral dimensions in the output plane.
It would also be desirable to provide electromagnetic radiation enhancement techniques employing amplitude and phase masks that can be used to adjust the shape of the high peak irradiance distributions in the output plane.
It would also be desirable to provide electromagnetic radiation enhancement techniques employing amplitude and phase masks that can be used to adjust the lateral position of the high peak irradiance distributions in the output plane that do not require traditional scanning mechanisms, and that are simpler to implement and will not introduce additional distortions to the electromagnetic radiation.
It would also be desirable to provide electromagnetic radiation enhancement techniques employing amplitude and phase masks that can produce high radiance distributions with low angular divergence of the electromagnetic radiation.
It would also be desirable to provide electromagnetic radiation enhancement techniques employing amplitude and phase masks that can be used to adjust the angular distribution of the electromagnetic radiation in the far field.
It would also be desirable to provide electromagnetic radiation enhancement techniques employing amplitude and phase masks that can be used to adjust the angular direction of the electromagnetic radiation in the far field of the optical system without mechanical scanning mechanisms.
Furthermore, it would also be desirable to provide optical systems for implementation of the above identified electromagnetic radiation enhancement techniques.